1 86 MODERN IDEAS OF EVOLUTION 



animal, and which are increased in number to suit its 

 increasing weight as it grows in size. There are 

 some reasons to believe that this entirely different 

 contrivance is as old in geological time as the cham 

 bered shell of the nautiloid animals. It was, indeed, 

 in all probability, more common and adapted to larger 

 animals in the Silurian period than at present. 



Another curious instance not, so far as yet known, 

 existing at all in the modern world, is that of the re 

 markable stalked star-fish described by Professor Hall 

 under the name Camerocrinus, and whose remains are 

 found in the Silurian rocks. The crinoids, or feather- 

 stars, are well-known inhabitants of the seas both in 

 ancient and modern times, but previous to Professor 

 Hall s discovery they were known only as animals 

 attached by flexible stems to the sea-bottom or 

 creeping slowly by means of their radiating arms. 

 It was not suspected that any of them had committed 

 themselves to the mercy of the currents suspended 

 from floats. It appears, however, that this was actually 

 realised in the Silurian period, when certain animals 

 of this group developed a hollow calcareous balloon- 

 shaped vesicle, from which they could hang suspended 

 in the water and float freely. So far as known, this 

 remarkable contrivance was temporary, and probably 

 adapted to some peculiarities of the habits and food 

 of these animals, occurring only in the geological 

 period in which they existed. 



Examples of this sort of adjustment are found in 



